tox.ini: rename `pep8` environment to `flake8`
tox.ini: rename `pep8` environment to `flake8`

That's the tool's name.

Signed-off-by: Philippe Proulx <2096628897b40c93960fdd9e24c9c883a54d4fe9@gmail.com>

efficios/barectf,efficios/barectf